1. Field of the Invention
The present invention relates to a method of manufacturing a high strength steel sheet which is generally used for inner and outer steel sheets of a structural member for an automobile, and more particularly, to ferrite-based high strength and lowgravity steel sheets, which have lower specific gravity and higher specific strength as compared to conventional high strength steel sheets, and have excellent ridging resistibility and ductility for preventing ridging from occurring due to severe machining such as drawing or the like, and a manufacturing method thereof.
2. Description of the Related Art
Recently, steel sheets for an automobile show a tendency toward more excellent formability owing to the complexity and integration of automobile moldings as well as gradual increase in requirements for strength and weight for the purpose of reduction in fuel expenses and improvement in safety upon collision of the automobile. Because of very excellent strength and ductility, and very low cost as compared to aluminum or magnesium, steel has been generally used to make a body of the automobile lightweight through thinning of a high strength and ductility steel plate up to now. However, in order to overcome a future limitation to the reduction in weight, it is inevitable to use nonferrous lightweight metal.
In the prior art, there was used steel to which a lightweight element, Al, is generally added so as to reduce specific gravity thereof. For example, there was proposed a method of manufacturing ferrite steel by adding 2.0 wt % to 10.0 wt % Al to very low carbon steel.
However, in the case of the ferrite steel, problems arise that it has only elongation of about 25%, and there is no solution to eliminate a so-called ridging phenomenon that upon machining such as deep drawing or the like, irregular linear striped defects are generated on the machined surface, so that the surface of the steel sheet becomes degraded, and the deep-drawn portion is broken.